The present invention relates to a system and method for tracking localizers. More particularly, certain embodiments of the present invention relate to an electromagnetic tracking system and apparatus used for tracking surgical instruments.
During surgical operations, it is beneficial to be able to track the direction and progress of a surgical instrument, such as a drill bit or screw, into a patient's body in order to ensure that the instrument is directed into the appropriate point in the body. Therefore, surgical tracking systems have been developed that are able to monitor and display movement of a surgical instrument relative to an image of the area of the patient's body where surgery is to take place. The area of the patient's body where surgery is to take place is imaged using an imaging technology such as an X-ray, MRI, CT scan or any other appropriate imaging technology. The scanned images are stored in a computer system and are displayed on a screen during the surgical procedure. The tracking system includes localizing devices on the instrument and patient that communicate with the computer system. The computer system translates the location of the localizing devices onto the image to recreate the relative position of the instrument to the patient.
Surgeons are able to track and predict the position of the instrument within the patient's body by viewing the position of the instrument on the image, and thus accurately align the drill bit or screw into the targeted area of the patient's body. By being able to use surgical tracking surgeons are able to effectively use surgical tools without having to take numerous X-rays during surgery to follow the progress of the surgical instrument. Therefore, operating room staff and patients are exposed to fewer X-rays for each surgical operation. Additionally, surgical tracking presents the potential for increased accuracy and improved surgical outcome.
There are two basic kinds of surgical tracking systems, optical tracking systems and electromagnetic tracking systems. Optical tracking systems use a number of emitters such as light emitting diodes (LEDs) or reflective spheres that are attached to the instruments. A camera system, also known as a digitizer, is used to track the positions of the emitters in space. The camera system is connected to the computer system which analyzes the positions of the emitters as recorded by the camera system to calculate the positions of the emitters, and thus the instruments, on the image.
Electromagnetic tracking systems use electromagnetic transmitters and receivers. The transmitter and receiver are in communication with the computer system. The transmitter generates an electromagnetic field and the receiver receives the field and generates signals to the computer based on the receiver's position in the electromagnetic field. The computer reads the signal from the receiver to calculate the position of the receiver relative to the transmitter. The transmitter is rigidly secured to the patient's body, by bone screws for example, proximate the area of the patient's body where surgery is to take place. A C-arm X-ray machine is connected to the computer system and takes at least one image of the area of the patient's body where surgery is to take place. The C-arm has a receiver that communicates with the transmitter and a calibration device that registers the position of the image relative to the C-arm. Thus, the computer system is able to calculate the position of points of the image relative to the transmitter. A surgical instrument has a receiver that communicates with the computer such that the computer calculates the receiver's position relative to the transmitter. The computer then calculates the position of the receiver relative to the image, and thus displays a representation of the instrument, properly positioned on the image based on these calculations. Alternatively, in some electromagnetic tracking systems, multiple transmitters and/or receivers are positioned relative to the instrument and the patient to track the instrument.
In electromagnetic systems, there are typically restrictions on the distances apart that a transmitter and receiver must be placed in order to yield accurate relative position information. The transmitter and receiver typically must not be too close to one another (e.g., less than a few inches) or too far apart (more than about 18 inches). Additionally, the presence of devices that generate electromagnetic fields, such as an electric drill, can cause interference with the accurate functioning of the localizing system. Likewise, the presence of some metals, such as those used in retractors, operating room tables or fluoroscopic C-arms, can cause interference that produces localizing errors. This interference is especially pronounced when the metal is in close proximity to either the transmitter or receiver, or anywhere in between them. This interference can be minimized by careful placement of the tracking devices relative to one another and to potential sources of interference.
The need to maintain the transmitter and the receiver in close proximity creates difficulties for certain procedures such as total knee surgery. Total knee surgery is an orthopedic procedure in which the articular cartilage of the knee is replaced with prosthetic metal and plastic components. In order to use surgical tracking in the placement of these prosthetic components, the transmitter typically needs to be attached proximate to the patient's knee. The proper positioning of the knee components typically requires that the joint centers of the hip, ankle and knee be collinear, defining the mechanical axis of the knee. One way to identify the joint centers is with x-ray images. However, when the C-arm X-ray machine is used to take images of the hip and ankle, and the transmitter is attached proximate the patient's knee, the distance between adjacent leg joints may be greater than the operating volume of the EM tracking system. Thus, the transmitter and receiver in the C-arm cannot effectively communicate such that the position of the transmitter relative to the image may be calculated. Therefore, images of the hip and ankle cannot be related to the surgical operation of the knee.
Additionally, other kinds of surgery may also involve distances too great to effectively use a conventional electromagnetic tracking system. In the repair of a fracture of the shaft of a long bone, a long rod (intramedullary rod or IM rod) is inserted down the central canal of the bone and rigidly holds the fracture fragments together. A first step in this procedure is the drilling of a hole in one end of the bone (the insertion end) and then inserting a long rod (reduction tool) down the canal to bridge the fracture and align the fragments. A final step in the procedure is the insertion of transverse locking screws into the bone and through holes in the IM rod at the end opposite the insertion end of the bone. During this surgery, the range of positions for a tracking sensor can vary widely. The tracking sensor on the reduction tool may be located at the opposite end from the tool's tip such that, as the tool is first inserted into the bone, the tracking sensor may be as much as 50 cm from the bone. During tracking of a drill to prepare the screw holes for the interlocking screws, the tracking sensor may be as much as 50 cm from the insertion end of the bone. Therefore, the working range for the tracker approaches one meter. Typically, such a distance is too great for a conventional surgical tracking system, especially an electromagnetic tracking system.
Other orthopaedic procedures may create similar difficulties for the surgical tracking system. Therefore, a need exists for an improved method and system for tracking the movement of an instrument relative to joints situated more than a certain distance from the area of surgery.